It is known that the storage proteins of wheat have a fundamental role in determining the nutritive, bread-making and dough-making properties of meal. The main storage proteins are classified as gliadins and glutenins on the basis of their solubility in aqueous solvents, molecular weight and amino acid composition.
Gliadins are monomeric molecules with a low molecular weight (30-70 Kilodaltons) which can be separated by electrophoresis on polyacrylamide gel in an acid buffer and are categorized into four groups (.alpha., .beta., .gamma. and .omega.) in decreasing order of mobility. On the basis of their N-terminal amino acid sequence, they are divided into three types .alpha., .gamma. and .omega.; the first two groups contain residues of cysteine which form intermolecular disulfide bonds, whereas .omega.-gliadins do not contain sulfurated amino acids.
Glutenins, on the other hand, are protein aggregates with a high molecular weight consisting of various polypeptide chains held together by intermolecular disulfide bonds. After reduction and separation by electrophoresis, they consist of subunits with a high (80-120 Kd) (HMW) and low (40-55 Kd) (LMW) molecular weight.
The high molecular weight subunits differ from gliadins not only in this characteristic but also in a greater content of glycine and a lower content of proline. The low molecular weight subunits resemble gliadins in their amino acid composition and molecular weight but are capable of forming intermolecular disulfide bridges and of binding themselves to the high molecular weight glutenin subunits, forming aggregates which are insoluble in alcohol solutions.
Although almost all the storage proteins of wheat participate in the formation of gluten, biochemical and genetic evidence shows that glutenin subunits with a low and high molecular weight play a fundamental role in determining the viscoelastic properties of gluten.
In particular, it has been shown that the presence of specific low molecular weight glutenins, called LMW-2, are responsible for the good qualitative characteristics of durum wheat (Pogna et al., (1990), J. Cereal Sci. 11, 15-34), whereas in soft wheat the qualitative characteristics are linked to the presence of the allelic pair of high molecular weight glutenins called Dx5-Dy10 (Payne et al. (1987)).
Analysis of the proteins of caryopses of wheat progeny deriving from intervarietal breeding and aneuploid lines has made it possible to localize the genes encoding the storage proteins on chromosomes and determine their heredity. In particular, the genes which encode glutenin subunits with a high molecular weight are localized on the long arms of chromosomes 1A and 1B, in complex loci indicated as Glu-1A and Glu-1B.
The low molecular weight glutenins are controlled by numerous genes situated on the Glu-3 loci of the short arm of chromosomes of group 1 of both genomes A and B and are strictly associated with the .gamma.-gliadin genes (Gli-1).
The genes which encode some high molecular weight proteins and .alpha., .beta. and .gamma. gliadins have recently been sequenced.
With respect to the genes which encode the low molecular weight glutenin subunits, analysis is limited to a few members of the family and in particular those which encode LMW-2 have not yet been isolated.